Title

Vibrational circular dichroism spectroscopy and its applications to conformational study of bifunctional chiral molecules

Date of Award

1995

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

Laurence Nafie

Keywords

Analytical chemistry, Chemistry

Subject Categories

Analytical Chemistry

Abstract

This dissertation is concerned with the solution conformations of selected bifunctional chiral molecules by experimental measurement and calculation of vibrational circular dichroism (VCD). In addition, the optical design for a Fourier transform vibrational circular dichroism (FT-VCD) instrument and improvements to this system have been evaluated. Included in the appendices of this dissertation are a comparison study of Fourier transform vibrational circular dichroism (FT-VCD) and multichannel detected Raman optical activity (ROA) and experimental VCD measurements of other chiral molecules.

The instrumental development study describes modifications for the FT-VCD instrument based on a Nicolet Magna 550 FT-IR spectrometer. The mirrors have been replaced by infrared lenses in the new optical designs for this system. High signal-to-noise (S/N) ratio, minimized artifacts and flat baselines have been achieved in raw VCD spectra for single enantiomer samples obtained on the upgraded FT-VCD instrument with much less collection time than before.

In the molecular conformation study, the OH-, NH- and CH-stretching absorbance and VCD spectra of (R,R)-(+)-hydrobenzoin (I), (1S,2R)-(+)-2-amino-1,2-diphenylethanol (II), (1R,2R)-(+)-1,2-diphenylethylenediamne (III), (S)-(+)-1-amino-2-propanol (IV) and (S)-(+)-2-amino-1-propanol (V), in dilute CCl$\sb4$ or $\rm C\sb2Cl\sb4$ solutions, have been measured. The mid infrared (IR) region absorbance and VCD spectra of molecules IV and V, in concentrated CDCl$\sb3$ solution, have also been recorded. Ab initio molecular orbital calculations of geometries, vibrational frequencies and VCD intensities far a number of conformers of these molecules have been carried out. The recently developed locally distributed origin gauge model (LDO) for VCD was used for first time to calculated the rotational strengths for the conformers of large size molecules I, II and III at a 3-21G basis set level. The calculate results have been found to give good agreement with experiment in the OH- and NH-stretching frequency regions. The solution conformations of this group of molecules have been identified through comparison of experimental and calculated results. The VCD spectra were interpreted based on previous research for small chiral diol, amino alcohol and amine molecules with the coupled oscillator model and the local chirality concepts.

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